Magnesium Based Composite Powder, Magnesium Based Alloy Base Material and Manufacturing Method Thereof

ABSTRACT

Magnesium based composite powder that is a starting raw material to manufacture a Mg 2 Si dispersion type of magnesium based composite material comprises Mg based powder ( 7 ) that is a main component constituting the matrix of a magnesium alloy, and Mg 2 Si powder attached to the surface of the Mg based powder ( 7 ) through a binder ( 9 ).

TECHNICAL FIELD

The present invention relates to a magnesium based composite powder containing Mg₂Si having high rigidity, a magnesium based alloy base material, and manufacturing methods of them.

BACKGROUND ART

A magnesium alloy is characterized by light in weight because of low specific gravity and commercialized and practically used as a package of a mobile phone and portable audio device mainly. In designing a product or a member, the rigidity of a base material is an important factor as well as strength and hardness.

For example, in a case where a magnesium alloy is applied to a housing case for an automatic transmission (AT), even when it has the same tensile strength and creep strength as that of a presently used aluminum alloy (ADC12, for example), since the rigidity (Young's modulus) of an existing magnesium alloy is about 60% of the aluminum alloy, it is bend and deformed when a load is applied in case of the same dimension and thickness. Therefore, when the magnesium alloy is practically used, since it is necessary to increase the thickness in accordance with a product and member to apply, the effect of its light in weight cannot be provided.

To improve the rigidity of a metal material including the magnesium alloy, it is effective in general to use a method of diffusing compound grains having rigidity higher than that metal material, that is, to use a composite material. For example, since magnesium silicide (Mg₂Si) has Young's modulus of 120 GPa and it is considerably higher than a general magnesium alloy having Young's modulus of 43 to 44 GPa, rigidity can be improved in a composite material in which its grains are dispersed in an alloy.

However, when a Mg—Si group alloy is manufactured by melting and casting methods, an eutectic point exists in the vicinity where a Si content is about 1% by weight. Thus, when Si is added far beyond 1% by weight, Mg₂Si formed by the reaction with Mg grows to be coarse. When the magnesium alloy contains such coarse Mg₂Si particles, strength and toughness are lowered due to stress concentration of the coarse particles, and explosion could be generated in the course of melting because heat is generated due to the reaction between Mg and Si. In addition, since the coarse Mg₂Si particles exist, performance in casting to a mold (casting property) is lowered and many defects and holes exist in the casting alloy base material. Thus, AS21 alloy (Mg-2% Al-1% Si) or AS41 alloy (Mg-4% Al-1% Si) and the like is used as Mg₂Si containing magnesium alloy that can be manufactured by the melting and casting method. However, when 1% by weight of Si is added, a volume ratio of the generated Mg₂Si particles is less than 1% of the total volume, it is difficult to improve the rigidity of the magnesium alloy satisfactorily.

Meanwhile, S. K. THAKUR and the like (adapted from Metallurgical and Materials Transactions A, Vol. 35A, March 2004, p. 1167-1176) proposes a manufacturing method of magnesium alloy containing Mg₂Si particles using an infiltration method in which a preform provided by solidifying mixed powder of three kinds including Si powder is prepared and it is impregnated with a melted magnesium alloy while a pressure is applied. However, according to this method, Mg₂Si particle formed by the reaction between a molten Mg alloy and Si grows in the course of reaction and becomes as coarse as 70 to 100 μm finally in the magnesium alloy. As a result, the above various problems arise in performance.

The inventor of the present invention disclosed a technique for manufacturing a magnesium based composite material in which Mg₂Si particles are diffused using a powder metallurgy method in Japanese Patent Application No. 2003-2602 (filed on Jan. 8, 2003). Here, the inventors proposes a magnesium composite powder in which fine Si powder or SiO₂ powder is attached on the surface of magnesium based alloy powder by a mechanical bonding method or a bonding method using a binder, and its manufacturing method. Furthermore, a warm plastic process is performed to the composite powder to generate Mg₂Si using the solid-phase reaction of Mg with Si or SiO₂, and to finally provide the magnesium based composite material in which Mg₂Si particles are uniformly dispersed.

According to the magnesium based composite material provided by the technique disclosed in Japanese Patent Application No. 2003-2602, high tensile strength is provided but it is necessary to heat to a high temperature (400 to 550° C., for example) for the reaction of Mg with Si or SiO₂. In this process, the magnesium crystal grain grows to be coarse. In other words, in order to implement further high strength, the heating temperature is preferably low, however, it is difficult to lower the temperature to about 300° C. in view of the above solid-phase reaction.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide high-rigidity and high-strength magnesium alloy containing a lot of fine Mg₂Si particles without high-temperature heating.

The inventor of the present invention has found that it is effective to use magnesium based composite powder in which Mg₂Si particles exist on the surface and/or inside of the Mg based powder when a magnesium based composite material in which Mg₂Si particles are dispersed is manufactured. The inventor has found that the following advantages can be provided by using Mg₂Si particles, instead of the method in which Mg₂Si particle is synthesized by the solid-phase reaction of Mg powder with Si grain as disclosed in Japanese Patent Application No. 2003-2602.

(1) Since heating to high temperature of 400 to 550° C. required for promoting the above Si—Mg reaction is not needed, the Mg crystal grain of a matrix is prevented from growing to become coarse and the strength of Mg based alloy is prevented from being lowered.

(2) Since heat generation in the course of the Si—Mg reaction is avoided, Mg₂Si particle and Mg crystal grain are prevented from becoming coarse.

That is, a magnesium based alloy can be manufactured through a warm extruding process at 200 to 400° C. without a high-temperature heat treatment for the solid-phase reaction between Mg and Si. As a result, there can be provided a high-rigidity and high-strength magnesium based alloy containing a lot of fine Mg₂Si particles.

A magnesium based composite powder according to the present invention comprises magnesium based powder, and magnesium silicide (Mg₂Si) that is dispersed on the surface and/or inside of the Mg based powder.

According to the magnesium based composite powder, the maximum particle diameter of the Mg₂Si particle is not more than 50 μm, preferably not more than 20 μm, and more preferably not more than 5 μm. In addition, it is preferable that the Mg based composite powder contains 5 to 60% by volume of Mg₂Si.

A magnesium based alloy base material according to the present invention is provided by compacting and sintering the magnesium based composite powder, and Mg₂Si particles are dispersed in the matrix.

According to one embodiment, a manufacturing method of magnesium based composite powder in the present invention comprises the following steps.

-   a) A step of preparing magnesium (Mg) based powder and magnesium     silicide (Mg₂Si) powder. -   b) A step of applying a binder to the surface of the Mg based     powder. -   c) A step of bonding the Mg₂Si powder to the surface of the Mg based     powder by mixing and stirring the Mg based powder to which the     binder has been applied and the Mg₂Si powder.

According to another embodiment, a manufacturing method of magnesium based composite powder in the present invention comprises the following steps.

-   a) A step of preparing magnesium (Mg) based powder and magnesium     silicide (Mg₂Si) powder. -   b) A step of blending the Mg based powder and the Mg₂Si powder and     mechanically mixing them to bond the Mg₂Si powder to the surface of     the Mg based powder mechanically.

According to the above methods, the method of bonding the Mg₂Si powder to the surface of the Mg based powder mechanically is such that the blended powder is mechanically mixed using a ball mill, a mixing and grinding mill, a roller compactor, or a rolling machine, for example.

According to still another embodiment, a manufacturing method of magnesium based composite powder in the present invention comprises the following steps.

-   a) A step of preparing magnesium (Mg) based powder and magnesium     silicide (Mg₂Si) powder. -   b) A step of mixing the Mg based powder and the Mg₂Si particles. -   c) A step of manufacturing a Mg based powder compact in which the     Mg₂Si particles are dispersed by compacting the mixed powder in a     mold. -   d) A step of manufacturing a Mg based sintered alloy in which the     Mg₂Si particles are dispersed by sintering the Mg based powder     compact. -   e) A step of powdering the Mg based sintered alloy by mechanical     grinding or cutting.

According to still another embodiment, a manufacturing method of magnesium based composite powder in the present invention comprises the following steps.

-   a) A step of preparing magnesium (Mg) based powder and silicon (Si)     powder. -   b) A step of mixing the Mg based powder and the Si powder. -   c) A step of manufacturing a Mg based powder compact in which the Si     particles are dispersed by compacting the mixed powder in a mold. -   d) A step of synthesizing Mg₂Si by heating the Mg based powder     compact to react Mg with Si, and at the same time manufacturing a Mg     based sintered alloy in which the Mg₂Si particles are dispersed. -   e) A step of powdering the Mg based sintered alloy by mechanical     grinding or cutting.

According to still another embodiment, a manufacturing method of magnesium based composite powder in the present invention comprises the following steps.

-   a) A step of inputting magnesium silicide (Mg₂Si) powder to molten     magnesium based alloy and stirring it. -   b) A step of manufacturing a casting base material by casting the     molten material in a mold. -   c) A step of powdering the casting base material by mechanical     grinding or cutting.

According to still another embodiment, a manufacturing method of magnesium based composite powder in the present invention comprises the following steps.

-   a) A step of preparing magnesium (Mg) based powder and magnesium     silicide (Mg₂Si) powder. -   b) A step of mixing the Mg based powder and the Mg₂Si powder. -   c) A step of manufacturing a Mg based powder compact in which the     Mg₂Si particles are dispersed by compacting the mixed powder in a     mold. -   d) A step of manufacturing a Mg₂Si particle dispersed Mg based     casting base material by dissolving and casting the Mg based powder     compact. -   e) A step of powdering the casting base material by mechanical     grinding or cutting.

According to still another embodiment, a manufacturing method of magnesium based composite powder in the present invention comprises the following steps.

-   a) A step of preparing magnesium (Mg) based powder and silicon (Si)     powder. -   b) A step of mixing the Mg based powder and the Si powder. -   c) A step of manufacturing a Mg based powder compact in which the Si     particles are dispersed by compacting the mixed powder in a mold. -   d) A step of synthesizing Mg₂Si by heating the Mg based powder     compact to react Mg with Si, and at the same time manufacturing a Mg     based sintered alloy in which the Mg₂Si particles are dispersed. -   e) A step of manufacturing a Mg₂Si particle dispersed Mg based     casting base material by dissolving and casting the Mg based     sintered alloy. -   f) A step of powdering the casting base material by mechanical     grinding or cutting.

A manufacturing method of a magnesium based alloy base material according to the present invention comprises a step of compacting the above described magnesium based composite powder, a step of heating the powder compact to 200 to 400° C. in an inert gas atmosphere or a non-oxygenated gas atmosphere, and a step of extruding the powder compact to a dense state immediately after the heating.

The characteristics and the working effect of the present invention will be described in the following.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing one example of a method of bonding Mg₂Si powder onto the surface of Mg based powder with a binder;

FIG. 2 is a schematic view showing another example of a method of bonding Mg₂Si powder onto the surface of Mg based powder with a binder;

FIG. 3 is a schematic view showing one example of Mg based composite powder in which the Mg₂Si powder are attached on the Mg based powder;

FIG. 4 is a schematic view showing another example of Mg based composite powder in which the Mg₂Si powder are attached on the Mg based powder;

FIG. 5 is a view showing one example of a method of manufacturing Mg based composite powder;

FIG. 6 is a schematic view showing one example of Mg based composite powder in which the Mg₂Si particles are dispersed in the matrix of the Mg based powder;

FIG. 7 is a view showing another example of the method of manufacturing the Mg based composite powder;

FIG. 8 is a view showing still another example of the method of manufacturing the Mg based composite powder;

FIG. 9 is a view showing still another example of the method of manufacturing the Mg based composite powder;

FIG. 10 is a view showing still another example of the method of manufacturing the Mg based composite powder;

FIG. 11 is a microscope photograph showing one example of a sectional structure of the Mg based composite powder in which the Mg₂Si particles are dispersed in the matrix of the Mg based alloy;

FIG. 12 is a view showing one example of X-ray diffraction of the Mg based composite powder; and

FIG. 13 is a view showing another example of X-ray diffraction of the Mg based composite powder.

BEST MODE FOR CARRYING OUT THE INVENTION

(1) Mg Based Composite Powder

(A) Mg₂Si Content

When it is assumed that the whole volume of magnesium based composite powder is 100%, 5 to 60% by volume of Mg₂Si is contained. In addition, in view of machine processability (machinability) of magnesium based alloy provided by solidifying the composite powder, a more preferable Mg₂Si content is 20 to 40% by volume. When the Mg₂Si content is less than 5%, the magnesium alloy does not have sufficient rigidity. On the other hand, when the Mg₂Si content is more than 60%, the Mg₂Si particles are segregated and aggregated in the magnesium based composite powder containing the Mg₂Si particles, so that the magnesium based alloy provided by solidifying that powder is low in strength and toughness. Thus, to provide the same level of rigidity as an aluminum alloy and to secure superior strength and machinability, a more preferable Mg₂Si content is 20 to 40% by volume.

(B) Maximum Particle Diameter of Mg₂Si

The maximum particle diameter of Mg₂Si contained in the magnesium based composite powder is to be not more than 50 μm, preferably not more than 20 μm, and more preferably not more than 5 μm. When the maximum particle diameter of the Mg₂Si particle is more than 50 μm, the mechanical property and machinability of the provided magnesium based alloy is lowered. When the value is not more than 20 μm, even if 40% by volume of Mg₂Si particles is contained, preferable machinability can be maintained. Furthermore, when the maximum particle diameter of the Mg₂Si particle is not more than 5 μm, the machinability of the magnesium based alloy is improved and at the same time tensile strength of that alloy is improved because of dispersion of fine Mg₂Si particles.

(C) Rigidity (Young's Modulus)

The Young's modulus of the magnesium based alloy is 48 to 90 GPa. When the Youngs modulus is less than 48 GPa, an increase rate of the existing magnesium alloy with respect to the Young's modulus becomes 10% or less, it is difficult to apply the alloy to a cover case associated part for a car or a package part of a personal computer or a portable device. On the other hand, when the Young's modulus is more than 90 GPa, since the Mg₂Si content is 60% or more by volume, the toughness and the machinability of the alloy base material are lowered as described above.

(2) Manufacturing Method of Magnesium Based Composite Powder

(A) Magnesium Based Composite Powder Using Bonding with Binder

FIGS. 1 and 2 show manufacturing methods of the Mg based composite powder using a binder solution, and FIG. 3 schematically shows the sectional structure of the Mg based composite powder provided by the methods.

According to the above methods, the composite powder is manufactured by a wet granulating machine or a spray dryer. According to the method shown in FIG. 1, a mixture 2 of Mg based powder and Mg₂Si powder is inputted into a container 1, and warm air 3 is supplied from the lower part of the container to float the mixture 2. In this state, a binder solution 4 is sprayed from the above to the mixture 2 to apply the binder to the surface of each powder particle and at the same time to dry it at high temperature. As a result, as shown in FIG. 3, Mg₂Si powder 8 is attached and bonded to the surface of a Mg based powder 7 through a binder 9.

According to the method shown in FIG. 2, in a state where the mixture of Mg based powder and Mg₂Si powder is floated by a relatively low air amount in the container 1, the binder solution 4 is perpendicularly sprayed with respect to the air flow direction from beneath.

In addition, although it is not shown, Mg₂Si powder can be attached and bonded to the surface of Mg based powder through a binder similarly by mixing and stirring the Mg₂Si powder in the binder solution and spray and applying the binder solution to the Mg based powder floated by the warm air.

Furthermore, according to another method, a predetermined amount of Mg based powder is inputted into the container and 0.2 to 0.5% by weight of olein acid serving as a binder is added to the Mg based powder and then, the olein acid is applied to the surface of the Mg based powder in the container by vibrating or rotating the whole container. Then, Mg₂Si powder is added into the container and the Mg₂Si powder is attached to the Mg based powder to which the olein acid has been applied by vibrating or rotating the container again. Thus, there is provided Mg based composite powder as shown in FIG. 3

(B) Magnesium Based Composite Powder Provided by Mechanical Bonding

Meanwhile, according to a method of mechanical bonding, Mg based powder and Mg₂Si powder are mixed and inputted to a ball mill, a mixing grinding mill, a roller compactor, a rolling machine and the like, and compressing and shearing processes and the like are applied to the mixture. As a result, there is provided a Mg based granulated substance in which the Mg₂Si powder is mechanically attached and bonded to the surface of the Mg based powder surface. According to need, Mg based composite powder having a predetermined dimension and configuration having a sectional structure as shown in FIG. 4 can be provided from this granulated substance using a grinding and sieving machine. According to Mg based composite powder 15 shown in FIG. 4, Mg₂Si powder 8 is mechanically bonded and attached to the surface of Mg based powder 7.

(C) Mg Based Composite Powder Using Mg₂Si Particle Dispersion Type of Mg Based Sintered Alloy

(a) Method Shown in FIG. 5

Mg based powder and Mg₂Si powder are prepared as a starting material and both are mixed at a predetermined blend ratio and stirred and inputted into a mold. Then, it is solidified by pressure, whereby a Mg based powder compact in which Mg₂Si particles are dispersed is manufactured.

When the above powder compact is heated to a temperature lower than a melting point of the Mg based powder in an inert gas or non-oxygenated gas or vacuum, a Mg₂Si particle dispersion type of Mg based sintered alloy is provided by solid-phase diffusion among Mg based powder.

When the Mg₂Si particle dispersion type of Mg based sintered alloy is powdered by mechanical processing such as grinding using a ball mill or a crusher mill or cutting, a Mg based composite powder 16 having a predetermined dimension and configuration in a sectional structure is provided as shown in FIG. 6. According to the Mg based composite powder 16 shown in FIG. 6, Mg₂Si particles are mainly dispersed in the matrix of Mg based powder 7.

In addition, in the above Mg₂Si particle dispersion type of Mg based sintered alloy, when the Mg₂Si content is more than 60% by volume, the Mg₂Si particles are segregated and aggregated and a problem in machinability in which a tool is damaged in the cutting process is generated. In view of the above, it is preferable that the Mg₂Si content is not more than 60%.

(b) Method Shown in FIG. 7

Mg based powder and Si powder are prepared as a starting material and both are mixed at a predetermined blend ratio and stirred and inputted into a mold. Then, it is solidified by pressure, whereby a Mg based powder compact in which the Si particles are dispersed is manufactured.

When the above powder compact is heated to a temperature lower than a melting point of the Mg based powder in an inert gas or non-oxygenated gas or vacuum, Mg₂Si is synthesized by a solid-phase reaction between Si—Mg and at the same time, a Mg₂Si particle dispersion type of Mg based sintered alloy is provided by solid-phase diffusion among Mg based powder.

When the Mg₂Si particle dispersion type of Mg based sintered alloy is powdered by mechanical processing such as grinding using a ball mill or a crusher mill or cutting, Mg based composite powder containing Mg₂Si particles and having a predetermined dimension and configuration in a sectional structure is provided as shown in FIG. 6.

(D) Magnesium Based Composite Powder Using Mg₂Si Particle Dispersion Type of Mg Based Casting Alloy

(a) Method Shown in FIG. 8

Mg₂Si powder prepared as a starting material are inputted to a molten Mg based alloy and stirred and cast into a mold. In the Mg based casting alloy taken out of the mold, added Mg₂Si powder is uniformly dispersed.

When the above casting alloy is powdered by mechanical processing such as grinding using a ball mill or a crusher mill or cutting, Mg based composite powder containing Mg₂Si particles and having a predetermined dimension and configuration in a sectional structure is provided as shown in FIG. 6. In addition, the dissolution temperature of the molten Mg based alloy after the Mg₂Si powder has been inputted is set to be less than the solidus line between Mg and Mg₂Si in a Mg—Si phase equilibrium diagram. Meanwhile, when it is heated to the solidus line temperature or more, Mg₂Si becomes a solid solution in the molten Mg based alloy and Mg₂Si grows to be coarse in the course of solidification after casting.

(b) Method Shown in FIG. 9

Mg based powder and Mg₂Si powder are prepared as a starting material and both are mixed at a predetermined blend ratio and stirred and inputted into a mold. Then, it is solidified by pressure, whereby a Mg based powder compact in which Mg₂Si particles are dispersed is manufactured. This powder compact is inputted into a crucible and heated to manufacture molten Mg based alloy in which Mg₂Si particles are dispersed. Then, it is sufficiently stirred and cast in a mold.

In a Mg based casting alloy taken out of the mold, the added Mg₂Si powder is uniformly dispersed and when this alloy is powdered by mechanical processing such as grinding using a ball mill or a crusher mill or cutting, Mg based composite powder containing Mg₂Si particles and having a predetermined dimension and configuration in a sectional structure is provided as shown in FIG. 6.

(c) Method Shown in FIG. 10

Mg based powder and Si powder are prepared as a starting material and both are mixed at a predetermined blend ratio and stirred and inputted into a mold. Then, it is solidified by pressure, whereby a Mg based powder compact in which Si particles are dispersed is manufactured.

When the above powder compact is heated to a temperature lower than a melting point of the Mg based powder in an inert gas or non-oxygenated gas or vacuum, Mg₂Si is synthesized by a solid-phase reaction between Si—Mg and at the same time, a Mg₂Si particle dispersion type of Mg based sintered alloy is provided by solid-phase diffusion among Mg based powder.

This powder compact is inputted into a crucible and heated to manufacture molten Mg based alloy in which Mg₂Si particles are dispersed. Then, it is sufficiently stirred and cast in a mold. In a Mg based casting alloy taken out of the mold, the added Mg₂Si powder is uniformly dispersed and when this alloy is powdered by mechanical processing such as grinding using a ball mill or a crusher mill or cutting, Mg based composite powder containing Mg₂Si particles and having a predetermined dimension and configuration in a sectional structure is provided as shown in FIG. 6.

In addition, since cutting oil used in the cutting process is attached to the Mg based composite powder, the above is to be used as a raw material after the cutting oil component has been removed by a cleaning process.

(3) Magnesium Based Alloy Base Material

The above Mg based composite powder containing the Mg₂Si particles as a starting raw material is solidified to provide a Mg based alloy in which the Mg₂Si particles are dispersed.

After a powder compact is provided from the Mg based composite powder, it is heated up and processed by extruding, forging or rolling. At this time, it is preferable that the heating temperature of the compact is about 200 to 400° C. When the heating temperature is lower than 200° C., the extruding process is hard to perform in some cases. On the other hand, when it is higher than 400° C., as the extruding process is speeded up, the temperature of the material after the extruding process is raised, which could lower the strength because of the course crystal grain.

In the Mg based alloy provided as described above, fine Mg₂Si particles are uniformly dispersed. Since the particle diameter of the Mg₂Si particle dispersed in the alloy is the same as the particle diameter in the Mg based composite powder, the maximum particle diameter of the Mg₂Si particle in the Mg based alloy is not more than 50 μm, preferably not more than 20 μm, and more preferably not more than 5 μm. In addition, the Mg based alloy contains 5 to 60% by volume of Mg₂Si. As a result, the Mg based alloy has high rigidity and high strength.

EXAMPLE 1

Pure Mg powder (purity is 99.9% and average particle diameter is 350 μm) and Si powder (purity is 99.9% and average particle diameter is 22 μm) were prepared and both powder were mixed at the ratio such that Mg:Si=2:1 (molar ratio) and a mixing process was performed with a ball mill for 30 minutes. The mixed powder was inputted into a carbon mold (inner diameter is 35 mm φ) and set in a spark plasma sintering machine and sintered for 15 minutes at a sample temperature of 600° C. under a pressure of 100 MPa in vacuum. As a result, a disk-shaped sample comprising Mg₂Si and having an outer diameter of 35 mm φ and a thickness of 12 mm was provided.

The disk-shaped sample was ground by a jet mill machine and finely ground and sieved to become a Mg₂Si powder having a maximum particle diameter of 15 μm or less, so that the Mg₂Si powder as a starting raw material was manufactured. On the other hand, as Mg based powder, AZ31 (nominal composition is Mg-3Al-1Zn/mass %) alloy powder having a diameter of approximately 2 mm was prepared.

Based on the method shown in FIG. 5, the AZ31 powder and the Mg₂Si powder were mixed at a predetermined ratio and inputted into a mold having a diameter of 60 mm φ. A pressure of 400 MPa was applied to it to manufacture a powder compact. This powder compact was sintered at 550° C. for 1 hour in a nitrogen gas atmosphere, whereby a AZ31 sintered alloy in which Mg₂Si particles are dispersed was provided. Then, Mg based composite powder having a diameter of 0.5 to 3 mm was manufactured from the sintered alloy by a cutting process.

FIG. 11 shows an observation result of the sectional structure of the composite powder when the Mg based composite powder contains 16% by volume of Mg₂Si particles. The Mg₂Si particles having a particle diameter of 15 μm or less are uniformly dispersed without segregating and aggregating, so that the Mg based composite powder according to the present invention was provided.

EXAMPLE 2

Based on the method shown in FIG. 7, pure Mg powder (purity is 99.9% and average particle diameter is 350 μm) and Si powder (purity is 99.9% and average particle diameter is 22 μm) were prepared and both powder were mixed at a predetermined ratio and a mixing process was performed with a ball mill for 30 minutes. The mixed powder was inputted into a mold having a diameter of 60 mm φ and a pressure of 400 MPa was applied to it to manufacture a powder compact.

When the powder compact is heat treated at 590° C. for 1 hour in vacuum, Mg₂Si particles are synthesized by the solid-phase reaction between Si and Mg, and at the same time, a Mg₂Si dispersed Mg sintered material was provided by promoting the sintering in Mg powder. Then, this sintered material was ground by a ball mill to manufacture Mg based composite powder having a diameter of 0.3 to 1 mm.

FIG. 12 shows a result of X-ray diffraction of the Mg based composite powder when the Mg based composite powder contains 7% by volume of Mg₂Si particles. Since only peaks of Mg and Mg₂Si are shown and there is no peak of Si used as the starting material, it is found that Si has completely reacted with Mg and consumed to synthesize Mg₂Si. In addition, since there is no peak of MgO, oxidation is not generated in the sintering process.

In addition, as a result of a structure observation by an optical microscope, it has been found that the average grain diameter of Mg₂Si dispersed in the powder material is about 24 μm and since it is the same as the particle diameter of Si powder of the starting raw material, no conspicuous coarse grain is not generated in the process of reaction with Mg. As a result, the Mg based composite powder in which the Mg₂Si particles are uniformly dispersed defined by the present invention was provided.

EXAMPLE 3

Pure Mg powder (purity is 99.9% and average particle diameter is 350 μm) and Si powder (purity is 99.9% and average particle diameter is 22 μm) were prepared and both powder were mixed at the ratio such that Mg:Si=2:1 (molar ratio) and a mixing process was performed with a ball mill for 30 minutes. The mixed powder was inputted into a carbon mold (inner diameter is 35 mm φ) and set in a spark plasma sintering machine and sintered for 30 minutes at a sample temperature of 600° C. under a pressure of 100 MPa in vacuum. As a result, a disk-shaped sample comprising Mg₂Si and having an outer diameter of 35 mm φ and a thickness of 18 mm was provided.

The disk-shaped sample was ground by a jet mill machine and finely ground and sieved to become a Mg₂Si powder having a maximum particle diameter of 10 μm or less, so that the Mg₂Si powder as a starting raw material was manufactured.

Based on the method shown in FIG. 8, AZ61 (nominal composition is Mg-6Al-1Zn/mass %) molten alloy was prepared in a carbon crucible. The above Mg₂Si powder is added at a predetermine ratio, keeping the molten temperature at 720 to 740° C. and sufficiently stirred. Then, it is cast in a mold to manufacture a AZ61 casting alloy base material in which the Mg₂Si particles are dispersed.

Thus, Mg based composite powder comprising AZ61 alloy and having a diameter of 0.5 to 3 mm was manufactured from the above casting alloy by a cutting process. Table 1 shows the Mg₂Si content (by volume) in the whole casting alloy material. In addition, the observation result of the sectional structure of the provided composite powder and damaged state of a super-hard tool after the cutting process to manufacture the powder are also shown in Table 1.

According to samples No. 1 to 5 that are examples of the present invention, since appropriate amount of Mg₂Si is contained, the Mg₂Si particles are uniformly dispersed in the matrix of the Mg based composite powder without segregating and aggregating. In addition, regarding tool abrasion (damaged state) generated when the Mg based composite powder is manufactured by the cutting process, although it is a little scraped, there is no problem.

Meanwhile, according to a sample No. 6 that is a comparative example, since the Mg₂Si content is as high as 65%, the Mg₂Si particles are aggregated in the powder matrix and since the casting alloy contains a lot of hard Mg₂Si, a tool is deeply damaged when the powder is manufactured by the cutting process and at the same time Mg is adhered at that damaged part. TABLE 1 Sample Mg₂Si content Dispersion state of Damaged state No. (by volume) Mg₂Si particles of a tool 1 7 Uniformly dispersed in Good surface the matrix 2 12 Uniformly dispersed in Good surface the matrix 3 28 Uniformly dispersed in Good surface the matrix 4 38 Uniformly dispersed in Very little scraped the matrix 5 57 Uniformly dispersed in Little scraped the matrix 6 65 Aggregation of Mg₂Si Severe damage and particles adherence of Mg

EXAMPLE 4

Pure Mg powder (purity is 99.9% and average particle diameter is 350 μm) and Si powder (purity is 99.9% and average particle diameter is 22 μm) were prepared and both powder were mixed at the ratio such that Mg:Si=2:1 (molar ratio) and a mixing process was performed with a ball mill for 30 minutes.

The mixed powder was inputted into a carbon mold (inner diameter is 35 mm φ) and set in a spark plasma sintering machine and sintered for 15 minutes at a sample temperature of 600° C. under a pressure of 100 MPa in vacuum. As a result, a disk-shaped sample comprising Mg₂Si and having an outer diameter of 35 mm φ and a thickness of 12 mm was provided.

The disk-shaped sample was ground by a jet mill machine. At that time, Mg₂Si powder having different diameters were manufactured by changing the grinding condition.

Based on the method shown in FIG. 9, as Mg based powder, AM60 (nominal composition is Mg-6Al-0.5Mn/mass %) alloy powder having a diameter of 3 mm was prepared as a starting material, and this and Mg₂Si powder was mixed at a predetermined ratio and inputted to a mold having a diameter of 60 mm φ and a pressure of 400 MPa was applied to manufacture a powder compact.

Then, the powder compact is inputted in AM60 molten alloy in a carbon crucible (molten alloy temperature is 720 to 740° C.) and sufficiently stirred and cast in a mold to manufacture a AM60 casting alloy base material in which Mg₂Si particles are dispersed. Then, Mg based composite powder (diameter is 0.5 to 3 mm) in which the matrix was AM60 alloy was manufactured from the casting alloy by a cutting process. In addition, the provided Mg based casting alloy contains 22% by volume of Mg₂Si.

In order to calculate the maximum particle diameter of the Mg₂Si particle dispersed in the Mg based composite powder matrix, the sectional structure of the composite powder was observed by an optical microscope and the maximum particle diameter of the Mg₂Si particle was found from the result by image analysis. The result is shown in Table 2. In addition, a damaged state of a super-hard tool generated in the cutting process when the Mg based composite powder is manufactured from the casting alloy is shown in Table 2.

According to samples No. 7 to 10 that are examples of the present invention, since the casting alloy contains the Mg₂Si particles having appropriate particle diameter, tool abrasion or damage is not generated when the powder is manufactured by the cutting process and a preferable surface property is provided.

Meanwhile, according to samples No. 11 and 12 that are comparative examples, since the maximum particle diameter of the Mg₂Si particle contained in the casting alloy is as large as 50 μm or more, a deep damage is generated in a tool at the time of cutting process and Mg is adhered to that damaged part. TABLE 2 Sample maximum particle diameter Damaged state No. of the Mg₂Si (μm) of a tool 7 3 Good surface 8 12 Good surface 9 26 Good surface 10 42 Good surface 11 75 Severe damage and adherence of Mg 12 92 Severe damage and adherence of Mg

EXAMPLE 5

The Mg based composite powder described in the above EXAMPLES 3 and 4 were prepared as starting materials and a powder compact was manufactured from each powder by molding. Each powder compact was heated and maintained at 350° C. for 5 minutes in a nitrogen gas atmosphere and immediately an extrusion process was performed to it (extrusion ratio 37) to manufacture an extruded base material. A tensile test piece was made from each extruded base material and its tensile characteristics (tensile strength and toughness) were evaluated at room temperature and its Young's modulus was measured. That result is shown in Table 3.

According to samples No. 1 to 5 and 7 to 10 that are examples of the present invention, there are provided magnesium based alloy having excellent strength and toughness and according to the samples No. 4 and 5 especially, they have rigidity equal to that of an aluminum alloy. In addition, as shown in the samples No. 7 and 8, when the maximum particle diameter of the Mg₂Si particle dispersed in the alloy is as fine as 5 μm or 20 μm or less, extension is considerably increased in addition to strength.

Meanwhile, according to a sample No. 6 that is a comparative example, since the Mg₂Si content is high, the Mg based alloy is fragile and it is difficult to manufacture a tensile test piece by machining. In addition, according to samples No. 11 and 12 that are comparative examples, since the maximum particle diameter of Mg₂Si particle is as large as 50 μm or more, the toughness of the Mg based alloy is lowered and its tensile strength is also lowered. TABLE 3 Tensile Young's Sample strength Toughness modulus No. (MPa) (%) (GPa) Remarks 1 278 18 49 Fragile and impossible 2 293 16 53 to make a test piece 3 314 13 63 4 322 9 70 5 326 5 82 6 — — 109 7 310 19 59 8 307 17 58 9 303 15 59 10 294 12 58 11 221 7 57 12 206 4 57

EXAMPLE 6

Pure Mg powder (purity is 99.9% and average particle diameter is 350 μm) and Si powder (purity is 99.9% and average particle diameter is 22 μm) were prepared and both powder were mixed at the ratio such that Mg:Si=2:1 (molar ratio) and a mixing process was performed with a ball mill for 30 minutes. The mixed powder was inputted into a carbon mold (inner diameter is 35 mm φ) and set in a spark plasma sintering machine and sintered for 30 minutes at a sample temperature of 600° C. under a pressure of 100 MPa in vacuum. As a result, a disk-shaped sample comprising Mg₂Si and having an outer diameter of 35 mm φ and a thickness of 18 mm was provided.

The disk-shaped sample was ground by a jet mill machine, and Mg₂Si powder having a maximum particle diameter of 10 μm or less and serving as a starting raw material was manufactured through grinding and sieving processes.

Then, 200 grams of pure Mg powder (purity 99%) having a particle diameter of 0.5 to 2 mm was inputted to a vinyl container having capacity of 350 ml and 0.6 gram of olein acid was added to the container and vibrated for 15 minutes by a vibration mill to apply the olein acid to the surface of the pure Mg powder in the container uniformly. Then, the Mg₂Si powder was added (13% by volume in the whole mixed powder) into the container and vibrated for 15 minutes to attach the Mg₂Si powder on the surface of the pure Mg powder, whereby Mg based composite powder defined by the present invention was provided.

FIG. 13 shows the result of X-ray diffraction of the provided Mg based composite powder. Only peaks of Mg and Mg₂Si that were inputted as raw materials are shown and it is found that fine Mg₂Si powder is uniformly attached onto the surface of the coarse pure Mg powder from the result of observation by a scanning electron microscope. As described above, it is confirmed that the Mg based composite powder can be manufactured using the olein acid as a binder.

Although the embodiments of the present invention have been described with reference to the drawings in the above, the present invention is not limited to the above-illustrated embodiments. Various kinds of modifications and variations may be added to the illustrated embodiments within the same or equal scope of the present invention.

INDUSTRIAL APPLICABILITY

According to the magnesium based alloy provided by the present invention, low rigidity that is a problem in performance of the conventional magnesium alloy can be considerably improved and the magnesium based alloy can be advantageously applied to a car component such as an engine part or a mission part or a structural member that requires high rigidity. 

1. A magnesium based composite powder comprising magnesium based powder, and magnesium silicide (Mg₂Si) that is dispersed on the surface and/or inside of the matrix of said magnesium based powder.
 2. The magnesium based composite powder according to claim 1, wherein the maximum particle diameter of said Mg₂Si is not more than 50 μm.
 3. The magnesium based composite powder according to claim 1, wherein the maximum particle diameter of said Mg₂Si is not more than 20 μm.
 4. The magnesium based composite powder according to claim 1, wherein the maximum particle diameter of said Mg₂Si is not more than 5 μm.
 5. The magnesium based composite powder according to claim 1, wherein the magnesium based composite powder contains 5 to 60% by volume of said Mg₂Si.
 6. A magnesium based alloy base material provided by compacting and sintering the magnesium based composite powder according to claim 1, wherein Mg₂Si particles are dispersed in the matrix.
 7. A manufacturing method of magnesium based composite powder comprising: a step of preparing magnesium (Mg) based powder and magnesium silicide (Mg₂Si) powder; a step of applying a binder to the surface of said Mg based powder; and a step of bonding the Mg₂Si powder to the surface of the Mg based powder by mixing and stirring the Mg based powder to which said binder has been applied and said Mg₂Si powder.
 8. A manufacturing method of magnesium based composite powder comprising: a step of preparing magnesium (Mg) based powder and magnesium silicide (Mg₂Si) powder; and a step of blending said Mg based powder and said Mg₂Si powder and mechanically mixing them to bond the Mg₂Si powder to the surface of the Mg based powder mechanically.
 9. A manufacturing method of magnesium based composite powder comprising: a step of preparing magnesium (Mg) based powder and magnesium silicide (Mg₂Si) powder; a step of mixing said Mg based powder and said Mg₂Si powder; a step of manufacturing a Mg based powder compact in which said Mg₂Si particles are dispersed by compacting said mixed powder in a mold; a step of manufacturing a Mg based sintered alloy in which said Mg₂Si particles are dispersed by sintering said Mg based powder compact; and a step of powdering said Mg based sintered alloy by mechanical grinding or cutting.
 10. A manufacturing method of magnesium based composite powder comprising: a step of preparing magnesium (Mg) based powder and silicon (Si) powder; a step of mixing said Mg based powder and said Si powder; a step of manufacturing a Mg based powder compact in which said Si particles are dispersed by compacting said mixed powder in a mold; a step of synthesizing Mg₂Si by heating said Mg based powder compact to react Mg with Si, and at the same time manufacturing a Mg based sintered alloy in which said Mg₂Si particles are dispersed; and a step of powdering said Mg based sintered alloy by mechanical grinding or cutting.
 11. A manufacturing method of magnesium based composite powder comprising: a step of inputting magnesium silicide (Mg₂Si) powder to molten magnesium based alloy and stirring it; a step of manufacturing a casting base material by casting said molten material in a mold; and a step of powdering said casting base material by mechanical grinding or cutting.
 12. A manufacturing method of magnesium based composite powder comprising: a step of preparing magnesium (Mg) based powder and magnesium silicide (Mg₂Si) powder; a step of mixing said Mg based powder and said Mg₂Si powder; a step of manufacturing a Mg based powder compact in which said Mg₂Si particles are dispersed by compacting said mixed powder in a mold; a step of manufacturing a Mg₂Si particle dispersed Mg based casting base material by dissolving and casting said Mg based powder compact; and a step of powdering said casting base material by mechanical grinding or cutting.
 13. A manufacturing method of magnesium based composite powder comprising: a step of preparing magnesium (Mg) based powder and silicon (Si) powder; a step of mixing said Mg based powder and said Si powder; a step of manufacturing a Mg based powder compact in which said Si particles are dispersed by compacting said mixed powder in a mold; a step of synthesizing Mg₂Si by heating said Mg based powder compact to react Mg with Si, and at the same time manufacturing a Mg based sintered alloy in which said Mg₂Si particles are dispersed; a step of manufacturing a Mg₂Si particle dispersed Mg based casting base material by dissolving and casting said Mg based sintered alloy; and a step of powdering said casting base material by mechanical grinding or cutting.
 14. A manufacturing method of a magnesium based alloy base material comprising: a step of compacting the magnesium based composite powder according to claim 1; a step of heating said powder compact to 200 to 400° C. in an inert gas atmosphere or a non-oxygenated gas atmosphere; and a step of extruding said powder compact to a dense state immediately after said heating. 